Look behind the largest chainring where each of the "nubs" are. If there are bolt heads at each nub location than you can probably remove the guard by removing each bolt at each "nub" with a 5mm allen wrench or whatever tool fits those bolts. The nubs should come off, too.

Otherwise, if there are no bolts connecting the guard to the chainring, given there's an opening you can probably pop that baby right off. If you need more leverage than your hands try gently prying it off by putting a large screwdriver (or something similar) between the guard and the chainring between the nubs and gently try to pry it loose. If it isn't budging with much effort, I'd leave it alone.

It's good to figure out how it's truly attached before going this route. A pic of the backside of the crank would probably tell the tale.